RFR: 8324972: (bf) Make DirectByteBuffer.Deallocator idempotent [v4]

[Per Minborg]

On Thu, 8 Feb 2024 10:58:05 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR proposes to make deallocators and unmappers for memory regions idempotent. This is to prevent (likely very rare) duplicate invocations. 
>> 
>> There are no unit tests but it should be noted that the idempotent behavior (now correct) is similar to the  intended behavior before cf74b8c2a32f33019a13ce80b6667da502cc6722 but where idempotency was not guaranteed in a multi-threaded environment.
>> 
>> Passes tier1, 2, and 3 tests.
>
> Per Minborg has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Use existing UNSAFE static variable
>  - Remove unused import

The https://bugs.openjdk.org/browse/JDK-8316337 was made to ensure visibility and safe publication between, on the one hand, the thread that allocated the resource and, on the other hand, the distinct cleaner thread.

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PR Comment: https://git.openjdk.org/jdk/pull/17647#issuecomment-1952627557